A number of lightning detection networks are currently in operation for detecting and locating cloud-to-ground (CG) discharges, i.e., discharges resulting in lightning strikes. The networks utilize a number of geographically dispersed sensors for sensing the RF emissions produced by the lightning.
It is also desirable to detect lightning strikes anywhere in the world, and so the sensors are likewise distributed across and around the globe. However, it is not practical to place sensors in many parts of the world, and it is costly to require more sensors than are needed, so it is of great practical advantage to utilize “long range” sensors. Such sensors are adapted to respond to ELF/VLF emissions (in the range of about 3 Hz-30 kHz), which are guided through the space between the earth's surface and the ionosphere, and therefore the sensors do not need to be within the line of sight of the strike. This provides a sensor range of up to about 10,000 km (about ¼ of the circumference of the earth).
To estimate the location of a lightning strike, lightning detection systems perform a triangulation based on data obtained from a number of geographically dispersed sensors. All but one prior art long-range lightning detection network triangulate based on what are known in the art as “time of arrival” (TOA) measurements. Each sensor has a GPS synchronized clock and determines the absolute time that a particular part of an RF emission is sensed. TOA data from a number of sensors are compared to obtain the difference in arrival time of the emission at the different sensors. Each time difference defines a hyperbola on which lie points of possible locations for the lightning strike. In general, N sensors define N−1 hyperbolas, and a minimum of three hyperbolas are necessary to triangulate a location unambiguously. For this reason, at least four sensors, positioned within a sufficient proximity to a strike to detect and measure the RF emissions, are generally necessary to provide the data needed to triangulate the position of the strike, and the use of additional sensors is highly desirable to improve the estimate.
One prior art detection network utilizes “angle” or “azimuth” measurements. The angle measurements are obtained from the ratio of outputs of a crossed loop magnetic field antenna. If angle measurements are accurate enough, triangulation can be performed using angle information only. However, angle measurements at long distances are known to have substantial errors, perhaps around 5 degrees. Where the sensor may be located up to 10,000 km from the lightning strike, a 5 degree error can produce an uncertainty in location of nearly 900 km. In long-range detection systems, a 5 degree error has been found to be unacceptable, with the result that triangulation is not performed with angle measurements. In the one prior art system that employs angle measurements, they are used as a supplement to TOA measurements.
In all the prior art systems, triangulation is performed at a “central analyzer” to which the sensors transmit their data over a communications channel for analysis.
In addition to determining the location of a lightning strike, it is considered important in the art to determine as well its peak amplitude and polarity. The emission produced by a lightning strike initially has a waveshape, polarity, and amplitude, and all of these characteristics of the wave change as the wave propagates. As a result of interaction of the wave with the surface of the earth and the ionosphere, by degrees as a function of the distance traveled, the wave becomes attenuated and more complex, and the polarity changes. The wave interacts with the earth differently over land than water, and interacts differently with the ionosphere depending on whether it is day or night. In TOA measurements, it is not possible to know that a particular point on the wave as it is initially produced maps or corresponds to a particular point on the wave as it is received, guaranteeing significant TOA measurement errors. It is likewise not possible to know how polarity as perceived at the sensor corresponds to initial polarity. So it is a severe drawback of prior art long-range lightning detection systems and methods that the emitted wave degrades over long distances to a point where it does not provide sufficient intelligible information about the original lightning strike to characterize the strike as desired.